In conventional power transmission devices for tractors, as shown in Patent Document 1, a main speed change mechanism that can switch between multiple speeds is provided between an input shaft to which forward and reverse drive power is transmitted from a forward/reverse drive switch mechanism, and a countershaft, and a sub-speed change mechanism that can switch between two speeds (a high and a low speed) is provided between the countershaft and an output shaft that transmits rear-wheel drive power. The driven gears on the countershaft of the main speed change mechanism are successively provided from front to rear in descending order of speed (the foremost driven gear is the highest-speed gear, and the rearmost driven gear is the lowest-speed gear). A sub-low-speed drive gear for the sub-speed change mechanism is provided between the second-speed and third-speed driven gears on the countershaft. The third-speed driven gear (next-highest-speed driven gear) is used as a sub-high-speed drive gear for the sub-speed change mechanism.
Clutch means is required in order for any one of a sub-low-speed driven gear and a sub-high-speed driven gear on the output shaft that are engaged with the sub-low-speed drive gear and the sub-high-speed drive gear (next-highest-speed driven gear), respectively, for the sub-speed change mechanism, to be selectively connected to the output shaft. The two driven gears need to be spaced from each other by an interval in an axial direction that is large enough to take the clutch means.
Patent Document 1 also describes the following. Specifically, a first and a second supporting wall are provided in a mission case linked to a flywheel housing to form a first chamber that is in communication with an interior of the flywheel housing, a second or middle chamber, and a third or rear chamber. The second and third chambers serve as an oil bath. A plurality of speed change mechanisms in a travel system that convert power of a propeller shaft into different speeds are provided in the second chamber. A front-wheel drive power extracting shaft that extracts power from the speed change mechanisms via front-wheel drive power extracting means in order to drive the front wheels, is supported by the second supporting wall that separates a rear portion of the second chamber.
In Patent Document 2, a first and a second supporting wall are provided in the front one of a front and a rear mission case. A hydraulic switch type forward/reverse drive switch mechanism that converts power of a propeller shaft into a forward speed and a reverse speed is provided in a second chamber. A main speed change mechanism that converts the power from the forward/reverse drive switch mechanism into multiple speeds, a sub-speed change mechanism that converts the power converted by the main speed change mechanism into a high and a low speed, and an output shaft that transmits rear-wheel drive power from the sub-speed change mechanism, are provided in a third chamber. A front-wheel drive power extracting shaft that extracts and transmits the power of the output shaft as front-wheel drive power via front-wheel drive power extracting means, is supported by the first and second supporting walls.
Patent Document 1 also describes the following. Specifically, a first and a second supporting wall are provided in a front mission case provided between a flywheel housing and a rear mission case to form a first chamber that is in communication with an interior of a flywheel housing, a second or middle chamber, and a third or rear chamber. A first speed change mechanism of a travel system that transmits power of a propeller shaft to the second chamber is provided in the second chamber. A second speed change mechanism that transmits power from the first speed change mechanism is provided in the third chamber. A PTO drive shaft is coaxially and directly linked to a rear portion of the propeller shaft. A PTO power transmission shaft is linked via a PTO clutch to a rear portion of the PTO drive shaft. A rear-axle supporting holder that forms a back wall of the third chamber is removably mounted to the front mission case. The rear-axle supporting holder supports a shaft of the second speed change mechanism. The PTO drive shaft penetrates the rear-axle supporting holder. Pump drive power extracting means that transmits power of the PTO drive shaft to a hydraulic pump mounted on an outer surface of the rear mission case is provided between the rear-axle supporting holder and the PTO clutch.
The pump drive power extracting means has a power transmission bevel gear mounted to the PTO drive shaft, and an extraction bevel pinion that is engaged with the power transmission bevel gear and is removably linked to a pump shaft of the hydraulic pump.